The present invention relates generally to the field of coherent signal detection, and more particularly to a digital coherent detector for directly sampling and digitizing an IF signal.
Receiving systems and radar receiving systems in particular, generally are designed to process their received signals at baseband in order to reduce the required A/D sampling rate. However, in order to obtain the information baseband, received intermediate frequency (IF) signals must be beat or heterodyned with a local oscillator intermediate frequency signal. Because the intermediate frequency signal could have any phase at the time of reception by the receiver, proper processing requires the generation of I and Q signals in phase quadrature in order to obtain phase and amplitude information. Phase accuracy, where the phase .0. is defined as Tan.sup.-1 Q/I, is dependent upon careful matching of these separate quadrature channels. Accordingly, phase errors limit the performance achievable from subsequent digital processing circuits such as MTI and DSLC. More specifically, the presently utilized method of forming two quadrature channels requires two separate frequency translations in parallel. The base-band mixers utilized in these frequency translations must track each other in order to obtain the desired phase accuracy. However, even a good pair of phase detectors for the I and Q channels will contribute a 2-4 degree error. Accordingly, such conventional baseband I/Q detection system will have a minimum of a 2-4 degree phase error which will inherently limit sidelobe cancellation ratios to about 30 dB. Thus, it can be seen that the phase error inherent in this quadrature channel approach presents significant limitations on radar signal processing.